RE: [PATCH v6 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

Chia-Yu Chang (Nokia)
2025-11-19T10:32:48Z

> -----Original Message-----
> From: Paolo Abeni <pabeni@redhat.com>
> Sent: Tuesday, November 18, 2025 2:59 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@nokia-bell-labs.com>; edumazet@google.com; parav@nvidia.com; linux-doc@vger.kernel.org; corbet@lwn.net; horms@kernel.org; dsahern@kernel.org; kuniyu@google.com; bpf@vger.kernel.org; netdev@vger.kernel.org; dave.taht@gmail.com; jhs@mojatatu.com; kuba@kernel.org; stephen@networkplumber.org; xiyou.wangcong@gmail.com; jiri@resnulli.us; davem@davemloft.net; andrew+netdev@lunn.ch; donald.hunter@gmail.com; ast@fiberby.net; liuhangbin@gmail.com; shuah@kernel.org; linux-kselftest@vger.kernel.org; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) <koen.de_schepper@nokia-bell-labs.com>; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire <cheshire@apple.com>; rs.ietf@gmx.at; Jason_Livingood@comcast.com; Vidhi Goel <vidhi_goel@apple.com>
> Subject: Re: [PATCH v6 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK
>
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> On 11/14/25 8:13 AM, chia-yu.chang@nokia-bell-labs.com wrote:
> > From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> >
> > For Accurate ECN, the first SYN/ACK sent by the TCP server shall set
> > the ACE flag (see Table 1 of RFC9768) and the AccECN option to
> > complete the capability negotiation. However, if the TCP server needs
> > to retransmit such a SYN/ACK (for example, because it did not receive
> > an ACK acknowledging its SYN/ACK, or received a second SYN requesting
> > AccECN support), the TCP server retransmits the SYN/ACK without the
> > AccECN option. This is because the SYN/ACK may be lost due to
> > congestion, or a middlebox may block the AccECN option. Furthermore,
> > if this retransmission also times out, to expedite connection
> > establishment, the TCP server should retransmit the SYN/ACK with
> > (AE,CWR,ECE) = (0,0,0) and without the AccECN option, while
> > maintaining AccECN feedback mode.
> >
> > This complies with Section 3.2.3.2.2 of the AccECN specification (RFC9768).
> >
> > Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> >
> > ---
> > v6:
> > - Use new synack_type TCP_SYNACK_RETRANS and num_retrans.
> > ---
> > include/net/tcp_ecn.h | 20 ++++++++++++++------
> > net/ipv4/tcp_output.c | 4 ++--
> > 2 files changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h index
> > a709fb1756eb..57841dfa6705 100644
> > --- a/include/net/tcp_ecn.h
> > +++ b/include/net/tcp_ecn.h
> > @@ -649,12 +649,20 @@ static inline void tcp_ecn_clear_syn(struct sock
> > *sk, struct sk_buff *skb) }
> >
> > static inline void
> > -tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr
> > *th) -{
> > - if (tcp_rsk(req)->accecn_ok)
> > - tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
> > - else if (inet_rsk(req)->ecn_ok)
> > - th->ece = 1;
> > +tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
> > + enum tcp_synack_type synack_type) {
> > + // num_retrans will be incresaed after tcp_ecn_make_synack()
>
> Please use /* */ for comments
>
> > + if (!req->num_retrans) {
>
> It's unclear you this function receives a `synack_type` argument and don't use it. Should the above be
>
> if (synack_type != TCP_SYNACK_RETRANS) {
>
> ?
>
> Or just remove such argument.
>
> /P
Hi Paolo,

You are right, and I will use both "synack_type != TCP_SYNACK_RETRANS" || "!req->num_retrans".

Because this ACE field fallback will only happen from the 2nd retansmitted SYN/ACK.

Chia-Yu